In most cases, the detection of the movement of an object in the form of a manually operable key of which the operation is to be detected, will be performed by mechanical switches comprising a switching element that is mechanically coupled to the key. Thus, when the key is operated, the switching element will be operated and the switching element in turn will establish a switching contact signaling the operation of the key. By way of alternative to such mechanical approaches, there also exist capacitive systems wherein the movement of an object is detected on the basis of a change of capacitance. Thus, referring to the example of a key of an operating element, actuation of the key will result e.g. in a change of the distance between two electrodes of the capacitor of a capacitive sensor, which can be evaluated with the aid of signal processing technology. To make it possible for the two electrodes to perform a relative movement, the volume between the two electrodes must be compressible and expandable, respectively. As a dielectric in this respect, gas or particularly air are suitable. In this regard, it is, however, disadvantageous that air, or gas in general, merely has a relatively low dielectric constant so that changes of capacitance caused by a change of the distance between the two electrodes will effect only small signal changes.
Diverse designs of capacitive sensors wherein use is made of changes of the electric coupling of the electrodes as are caused as a result of an effective force, are described in DE 10 2007 001 086 A1, EP 1 261 845 B1 (as well as U.S. Pat. No. 6,683,780 B2) and U.S. Pat. No. 5,206,785. In the sensor according to DE 10 2007 001 086 A1, use is made of a fixed dielectric, with an electrically conductive, flexible layer arranged at a distance therefrom. When a pressure is applied, a plunger will act on this layer and will move it toward the dielectric and into a more or less large surface contact with the dielectric, thus effecting the desired signal variation of the outputs signal of the sensor. Also in the sensor according to U.S. Pat. No. 5,206,785, a fixed dielectric is used. In EP 1 261 845 B1 and EUS 6 683 780 B2, a capacitive differential sensor with air as a dielectric is described.